Generally, in vehicles such as passenger vehicles, speakers are provided at a plurality of positions. For example, a left front speaker and a right front speaker are provided at positions symmetrical to each other with respect to a central line of an in-vehicle space. However, if a listening position of a listener (a driver seat, a front passenger seat, a rear seat or the like) is considered as a reference position, these speakers are not positioned symmetrically. Therefore, due to differences in distances between the listening position of the listener and each of a plurality of speakers, sound image localization bias by Haas effect occurs.
For example, Japanese Patent Provisional Publication No. H7-162985A (hereinafter, “Patent Document 1”) discloses an apparatus that is capable of remedying the sound image localization bias. The apparatus disclosed in Patent Document 1 suppresses the sound image localization bias by adjusting time such that playback sounds emitted from all of the speakers reach the listener at the same time (i.e., a time alignment process). More specifically, the apparatus disclosed in Patent Document 1 corrects, over the entire range, the sound image localization bias and frequency characteristic disorders due to phase interferences by dividing an audio signal into a high range and a low range using a band dividing circuit and then adjusting time of each of playback sounds to be emitted from each of low band speakers and high band speakers.
However, the apparatus disclosed in Patent Document 1 has a problem that linearity of transmission characteristic at the listening position of the listener degrades due to loss of signals and double additions that occur in the band dividing circuit. Furthermore, the apparatus disclosed in Patent Document 1 also has a problem that peaks and/or dips occur in frequency characteristic around a crossover frequency when mixing the signals divided by the band dividing circuit.
In view of above, a brochure of International Patent Publication No. WO2009/095965A1 (hereinafter, “Patent Document 2”) proposes an apparatus for performing a time alignment process that is capable of improving linearity of transmission characteristic at the listening position of the listener and suppressing occurrence of peaks and/or dips in frequency characteristic when mixing.